1. Field of the Invention
This invention relates to the fabrication of doped epitaxial layers of semiconductor materials. More particularly, method is provided for doping of the epitaxial layer by directing a supersonic beam carrying dopant atoms onto the epitaxial layer during growth.
2. Description of Related Art
Development of alternative semiconductor materials for electronic and optical devices is of interest because silicon and gallium arsenide cannot currently meet some of the power, frequency, temperature and speed requirements of the next generation of electronic and optical devices. Silicon carbide (SiC) has many of the required properties, such as wide bandgap, high electric field breakdown, high thermal conductivity, high saturated electron drift velocity and good physical strength, that make it attractive for many of these applications. There are now commercially available reproducible single crystal wafer SiC substrates in the 6H and 4H polytypes. As substrate quality continues to improve, more devices will be fabricated from this material. Among these devices are high-power switches, high-voltage high-frequency devices for cellular telephone communications and high definition television transmission, high-temperature electronics, blue emitters for high-density data storage and other devices.
One of the most important challenges that needs to be overcome for the realization of advanced electronic devices using SiC is control over the electronic properties of the epitaxial layers grown. This requires control over the doping of SiC. Silicon carbide crystals are grown either by sublimation ("Lely process") or by chemical vapor deposition (CVD). Post-growth doping techniques are not effective in SiC crystals. Ion implantation, known for some doping applications (U.S. Pat. No. 5,354,584), causes crystal damage which can only be removed by annealing, and the annealing results in excessive dopant redistribution. Therefore, doping during sublimation or CVD growth of epitaxial layers is the process likely to be used for commercial production of SiC devices. Unintentional n-type doping by nitrogen incorporation during CVD-growth of epitaxial layers remains a problem. Until recently, doping levels have been confined to the range of N.sub.d .congruent.2.times.10.sup.16 cm.sup.-3 to 5.times.10.sup.18 cm.sup.-3 for n-type carriers and N.sub.a .congruent.2.times.10.sup.16 cm.sup.-3 to 1.times.10.sup.18 cm.sup.-3 for p-type carriers. (D. J. Larkin et al, Appl. Phys. Lett. 65 (1994), 1659)
In recent years, control over doping of CVD-grown layers has been improved using a process called site competition epitaxy. (U.S. Pat. No. 5,463,978). Site competition epitaxy is a dopant control technique based on adjusting the Si/C ratio within the growth chamber to control the amount of dopant incorporated into substitutional SiC crystal lattice sites. These sites are either carbon lattice sites (C-sites) or silicon lattice sites (Si-sites) located on the active SiC growth surface. This process is based on the principle of competition between nitrogen and carbon for the C-sites and between aluminum and silicon for the Si-sites of the growing silicon carbide epitaxial layer. The concentration of n-type (nitrogen) dopant atoms incorporated into a growing silicon carbide epitaxial layer is decreased by increasing the carbon-source concentration, so that C out-competes N for the C-sites. Analogously, the amount of p-type dopant (aluminum) incorporated into the growing silicon carbide layer is decreased by increasing the silicon-source concentration within the growth chamber so that Si out-competes Al for the Si-sites. This process has been used to dope SiC from a p-type doping level of &lt;1.times.10.sup.14 cm.sup.-3 to n-type doping level of&gt;1.times.10.sup.19 cm.sup.-3.
The use of site-competition epitaxy, relying on changing the C/Si ratio to control the doping level, affects other properties of the SiC growth, such as growth rate and crystal quality. These other changes could have detrimental effects on a device. In addition, it is very difficult to rapidly change the C/Si ratio during growth--making abrupt doping level changes difficult to obtain.
The use of supersonic jets for thin film deposition has recently been studied experimentally (K. A. Pacheco et al, "Growth and characterization of silicon thin films employing supersonic jets," J. Vac. Sci. Technol. A 15(4), July/August 1997) and by theoretical analysis (G. Chen et al, "Monte Carlo analysis of a hyperthermal silicon deposition process," J. Vac. Sci. Technol. A 16(2), March/April 1998). With this technique, epitaxial layers are formed by seeding precursor molecules in a supersonic beam of light carrier gas. The precursor molecules are accelerated to the high velocity of the beam by the light carrier gas and then impinge at a high energy on a substrate, resulting in a greatly enhanced epitaxial growth rate compared with conventional CVD deposition. Supersonic jet molecular beams were originally suggested as a replacement for effusive gas sources where the supersonic jet of gas travels through a nozzle in the intended beam direction of the effusion source. Using supersonic beams removes the effect of the effusion constraint (Knudsen number &gt;1) on the mean free path of the molecular beam. The advantages of this approach are beam intensities on the order of 1000-times those available from effusive sources, extremely long mean free path, and narrow energy spread in the beam as a result of cooling of the gas by isentropic expansion. Most of the studies of supersonic beams have been applied to the growth of silicon, but heteroepitaxial growth of SiC on silicon by pulsed supersonic free jets has been investigated (Y. Ikoma et al, "Epitaxial growth of 3C--SiC on Si by pulsed supersonic free jets . . . ," J. Vac. Sci. Technol. A 16(2), March/April 1998; K. D. Jamison et al, "Seeded pulsed supersonic molecular beam growth of silicon carbide thin films," J. Vac. Sci. Technol. A 16(3), May/June 1998).
The properties of supersonic beams are described in the book Atomic and Molecular Beam Methods (ed. G. Scoles, 1988), especially in Vol. 1, Ch. 2. The energy of dopant molecules in a supersonic beam can be changed using a number of methods that either change the terminal velocity of the beam or change the mass ratio between the seed (dopant) molecule and the carrier gas in the beam. For a supersonic beam to impinge on a substrate at high velocity, it is necessary that collisions between the beam molecules and resident gas around the substrate be practically eliminated. Therefore, it is necessary that pressure in the growth chamber be reduced. The configuration of the beam, any "skimmer" or beam-defining device, the substrate, and the nozzle through which the gases enter a growth chamber are predictable using the theory presented in the book and by other calculation methods such as the Chen et al paper referenced above.
While growth of SiC devices is known by sublimation or by CVD and incorporation of dopants into the growing crystal is known, better methods are needed for controlling dopant levels in SiC. Better methods for controlling dopant levels are also needed for growth of other semiconductor materials. The methods should make possible change of dopant level without change of another growth variable which could affect crystal quality and have detrimental effects on a device.